Yars: Recharged

The swirl is that spinny thing that the cores fire from time to time after charging up. You should get this naturally just by playing the game, but if you don't, there are only two ways to destroy those (as far as I'm aware). Either fire a rocket at it, after getting the rocket power-up from a rocket tower, or use the Zorlon Cannon. If you want to farm this, just recharge the Zorlon cannon and hang around hear it. Once the core starts charging, fly into the cannon and stand by, prefereably at the same height as the core so it'll fire straight towards you. Once it does, fire your shot into the swirl and you're good.